The present invention relates to proximity switches of the type capable of wirelessly detecting a target in proximity with the proximity switch. The invention also relates to a target for use in combination with such a proximity switch, to a system of such proximity switches and targets, and to a method of determining the presence of a target by using a proximity switch.
Proximity switches are used in various applications for detecting the presence or absence of a second object in relation to a first object. For this purpose, the first object is provided with a proximity switch, and the second object is provided with a target, which may be detected by the proximity switch. For instance, proximity switches are commonly used in industrial and potentially dangerous environments for assisting in ensuring operational safety of a machine or an industrial process. To this end, a combination of a proximity switch and a target is used for detecting whether a gate, door, hatch, etc, is safely closed in a secure position, wherein a human operator is prohibited from directly accessing a potentially dangerous machine or process, or whether the gate, door, hatch, etc, is suddenly opened.
It is previously known to use mechanical or semi-mechanical relays or switches, for instance comprising permanent magnets and Reed switches, for monitoring gates, doors or hatches as described above. However, such switches are less desirable for various reasons. For instance, mechanical or semimechanical switches have various recesses, slots, etc. in which dirt may be accumulated. For sanitary reasons, this is not acceptable in several industrial fields, such as the food industry, the semiconductor industry or the pharmaceutical industry. Furthermore, such prior art proximity switches may be adversely affected by an external spurious magnetic field.
WO92/03876 relates to an inductive type of proximity switches, where the switch includes an open-ended loop adjacent to a coil of a parallel resonant (oscillator) circuit. A high-speed MOSFET transistor is connected between the ends of the loop, and a pulsed input to the transistor causes sequential making and breaking of a connection between these ends, wherein the loop will function as a Faraday shield to inhibite mutual inductive linking of aforesaid coil with a coil of a passive resonant circuit of a target, when said target is close to the proximity switch and the connection between the ends of the loop is made by the MOSFET transistor- The proximity switch operates by transmitting an inductive signal, which is intercepted by the passive resonant circuit of the target, wherein the critical resonant frequency of the oscillator circuit of the proximity switch will be lowered. By providing a pulsed input to the MOSFET transistor, the transistor will periodically act as an active shield, which inhibits the aforesaid influence from the passive resonant circuit of the target on the oscillator circuit of the proximity switch. Consequently, the voltage of the oscillator circuit will be lowered and raised periodically in response to the pulsed input signal to the MOSFET transistor
The proximity switch of WO92/03876 has a severe drawback in that it is susceptible to a spurious external magnetic field, which may be picked up by the oscillator circuit of the proximity switch in response to the pulsed input signal to the MOSFET transistor, wherein the spurious external magnetic field will be incorrectly interpreted by the proximity switch as an indication that the target is present, even if it is not.
In one embodiment of WO92/03876 a handshaking arrangement is established for an inductive type proximity switch and its associated target. In response to receiving a signal from the proximity switch, the target is arranged to transmit to the proximity switch a predetermined pulsed signal comprising a coded set of information. The target is given its individual code once and for all, before the target is initially put into service A controller of the proximity switch monitors the received coded pulsed signal transmitted by the target, and in response thereof the controller determines whether or not the target is present in proximity of the proximity switch.
This embodiment has a drawback in that it requires a fixed, predetermined code to be programmed into the target. The code is set initially and may not be changed at a later stage. However, it is often desired to provide a cascade of several pairs of proximity switches/targets connected in series with each other, so that an output of a first proximity switch is connected to an input of a subsequent proximity switch, thereby allowing a chain of movable objects (doors, hatches, gates, etc) to be monitored in an efficient manner. However, since the code of the target is fixed and set once and for all, it may be difficult to use the proximity switches of WO92/03876 in cascade, since individual proximity switches/targets thereof may have identical codes, which may interfere with each other.
It is an object of the present invention to provide a proximity switch and an associated target, which remedy the drawbacks of the prior art solutions. In particular, it is an object of the present invention to allow a cascade of proximity switches/targets to be connected in series with each other in an efficient and reliable manner. A further object is to provide a proximity switch and a target, which are more flexible and more reliable than previously known approaches.
Yet another purpose of the present invention is to provide a proximity switch, which may be connected to and monitored by any conventional safety relay commercially available on the market.
According to the present invention, the above objects are achieved by a proximity switch, a target, a system of proximity switches and targets, and a method according to the enclosed independent patent claims.